This invention relates to a photothermographic material and more particularly, to a photothermographic material capable of forming a high transparency image faithful to exposure, especially suited for the manufacture of printing plates.
Nowadays, scanners and image setters which can be exposed by means of lasers and light-emitting diodes find widespread use as output devices in the printing art. There is a strong demand for a printing photosensitive material having high sensitivity, Dmax, contrast, and image quality. On the other hand, it is strongly desired from the standpoints of environmental protection and space saving to reduce the amount of waste solution used in the processing of conventional photographic silver halide photosensitive materials.
However, printing photosensitive materials which can form images through a simple process without resorting to solution system processing chemicals have never been supplied to the customers.
Photothermographic materials which are processed by a photothermographic process to form photographic images are disclosed, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, D. Morgan and B. Shely, "Thermally Processed Silver Systems" in "Imaging Processes and Materials," Neblette, 8th Ed., Sturge, V. Walworth and A. Shepp Ed., item 2, 1969.
These photothermographic materials have been used as microphotographic and medical photosensitive materials. Only a few have been used as a graphic printing photosensitive material because the image quality is poor for the printing purpose as demonstrated by low maximum density (Dmax) and soft gradation.
Photothermographic materials having high Dmax and contrast are prepared by adding hydrazine derivatives to photosensitive material as disclosed in U.S. Pat. No. 5,496,695 and Japanese Patent Application No. 215822/1996. This photothermographic material has the undesirable tendency of images in exposed areas to thicken and large dots to collapse when exposed by means of a laser image setter. Since currently available advanced laser image setters insure high precision exposure, a photosensitive material capable of reproducing an image faithful to exposure is strongly desired.
When it is desired to produce high resolution images faithful to exposure in these photosensitive materials, one solution is by adding an anti-irradiation dye or providing an anti-halation layer as in conventional wet process photographic silver halide photosensitive materials. In the wet system, the anti-irradiation dye is added to the photosensitive layer while the anti-halation layer is disposed between the support and the photosensitive layer or on the surface of the support remote from the photosensitive layer. In the wet system, the dye is readily bleached or dissolved away during development. In photothermographic material using a dry process, such a dye cannot be fully removed, resulting in an image with residual color.
In the above-referred U.S. Pat. No. 5,496,695, reference is made to thermal decolorization of polymethine dyes having a specific structure (U.S. Pat. No. 5,135,842) and thermal decolorization of similar polymethine dyes by carbanion generators (U.S. Pat. No. 5,314,795). These dyes undesirably lose their anti-irradiation or anti-halation function as the photosensitive material is naturally aged or exposed to high temperature. The above-mentioned polymethine dyes of a specific structure have the problem that after decolorization, decomposition products of the dye are left to provide light absorption, especially in the UV region. There have been available no anti-irradiation or anti-halation dyes which can be used in high Dmax, ultrahigh contrast photothermographic material for use in the manufacture of printing plates.
These photothermographic materials generally contain a reducible silver source (e.g., organic silver salt), a catalytic amount of a photocatalyst (e.g., silver halide), a toner for controlling the tonality of silver, and a developing agent, typically dispersed in a binder matrix. Photothermographic materials are stable at room temperature. When they are heated at an elevated temperature (e.g., 80.degree. C. or higher) after exposure, redox reaction takes place between the reducible silver source (functioning as an oxidizing agent) and the developing agent to form silver. This redox reaction is promoted by the catalysis of a latent image produced by exposure. Silver formed by reaction of the organic silver salt in exposed regions provides black images in contrast to unexposed regions, forming an image.
Such photothermographic materials have been used as microphotographic and medical photosensitive materials. However, only a few have been used as a graphic printing photosensitive material because the image quality is poor for the printing purpose as demonstrated by low maximum density (Dmax) and soft gradation.
A photothermographic material having high Dmax and high contrast can be obtained by adding a hydrazine derivative to a photosensitive material as disclosed in Japanese Patent Application No. 228627/1995. When this photothermographic material having high Dmax and ultrahigh contrast is subject to high precision exposure which is enabled by the advanced laser technology, there arises a problem that image quality is exacerbated at dot edges.
With the recent advance of lasers and light-emitting diodes, scanners and image setters having an oscillation wavelength of 600 to 800 nm find widespread use. There is a strong desire to have a high contrast photosensitive material which has so high sensitivity and Dmax that it may comply with such output devices.
For the purposes of easy handling and rapid accurate feed of sheets of photosensitive material, the existing exposure apparatus and automatic processors which are used with conventional photographic silver halide photosensitive materials are sometimes equipped with a mechanism for detecting the photosensitive material. The detector is generally an optical sensor comprising a light source and a light receiving element. Light used for detection should have a wavelength in the wavelength region to which the silver halide in the photosensitive material is insensitive, typically the infrared region of 850 to 1,400 nm. Conventional photographic silver halide photosensitive materials have sufficient light absorption in the IR region to enable detection.
In contrast, photothermographic materials comprising an organic silver salt, silver halide and reducing agent on a support do not have sufficient light absorption to enable detection because the coverage of silver halide is extremely small (typically a silver coverage of less than 3 g/m.sup.2 even when combined with organic acid silver salt) as compared with conventional photographic silver halide photosensitive materials. Then with respect to film detection, the photothermographic materials do not comply with the existing exposure apparatus which are used with conventional photographic silver halide photosensitive materials.
It would occur to those skilled in the art that this problem is solved by adding dyes having light absorption in the IR region to photothermographic materials. In the wet system, such dyes are readily bleached during development or dissolved away in the processing solution and it never happens that an image becomes unclear due to the residual dye. In the photothermographic materials, however, the dyes cannot be dissolved away because of a dry process.
Known methods for diminishing the color of a dye without dissolving the dye away are by thermally decolorizing polymethine dyes having a specific structure as disclosed in U.S. Pat. No. 5,135,842 and by thermally decolorizing similar polymethine dyes using carbanion generators as previously mentioned. These dyes, however, are not IR absorbing dyes and tend to lower their concentration as the photosensitive material ages or is exposed to high temperature. The antihalation dyes described in JP-A 13295/1995 are added to photosensitive material as a solution in a good solvent and do not have sufficient IR absorption for detection.